1. Field of the Invention
The present invention relates to an electrophotographic photosensitive member and, more particularly, to an electrophotographic photosensitive member with excellent potential characteristics, having an electroconductive intermediate layer capable of reducing black spot fogging in images at high temperature and humidity.
2. Description of the Related Art
An electrophotographic photosensitive member is basically formed of a base and photosensitive layers. However, when the base is an insulating material, such as paper or plastic, an electroconductive film must be provided on the base in order to cause electrical charge to flow. When the base is a metal, such as aluminum, copper, brass, or stainless steel, an electroconductive film need not be formed on the base, but forming such a electroconductive film is effective for increasing the coating ability of the photosensitive member, protecting the photosensitive member against electrical breakdown, covering defects on the surface of the base, and the like. It is required that the coated layer have a sufficiently low electrical resistance such that it prevents electrical charges from being accumulated when it is used repeatedly in a high-speed electrophotographic process and that it provides stable potential characteristics.
Since it has been difficult in the past to obtain an electroconductive film described above by using only a single resin, usually the film is formed by dispersing electroconductive powder in a binder resin. As disclosed in, for example, Japanese Patent Laid-Open No. 61-163346, metallic powder, such as nickel, copper, silver, or aluminum; metallic oxide powder, such as iron oxide, tin oxide, antimony oxide, or indium oxide, or a mixture of these; carbon black; fibrous carbon, or the like, are used for such electroconductive powder.
An electrophotographic photosensitive member having a vapor deposition film containing indium oxide in which tin or tin oxide, or a mixture of both, are doped, is disclosed in Japanese Patent Laid-Open No. 52-7242.
However, the above-described electroconductive powder has certain drawbacks. Metallic powder, such as nickel, copper, silver, or aluminum, has sufficient electroconductivity. However, since it is comparatively easy to oxidize, the potential characteristics thereof are readily changed when it is used continuously at a high temperature and high humidity Accordingly, image defects, such as spot fog, is likely to occur. Also, electroconductive powder, such as electroconductive iron oxide, tin oxide, antimony oxide, titanium oxide, or a mixture of these has a comparatively high work function If the resistance thereof is sufficiently decreased so that residual potential is not accumulated because of repeated use at low temperature and low humidity, satisfactory potential characteristics can be obtained. On the other hand, at high temperature and high humidity, there is a drawback in that spot fog occurs
It is known that indium oxide has a low work function and is highly stable even in an oxide atmosphere or a reducing atmosphere. This fact is disclosed in, for example, Japanese Patent Laid-Open Nos. 1-233458, 3-136064, 3-136063, and 3-136062. However, indium oxide powder has a drawback in that its resistance is high. Accordingly, characteristics required for a photosensitive member; i.e., sensitivity, residual potential and repeatability under different environmental conditions, cannot be satisfied unless a considerably increased amount is employed in order to form a practical photosensitive drum. Also, the resistance of a vapor deposition film containing indium oxide in which tin or tin oxide, or a mixture of both of these, are doped, is low. For this reason, a vapor deposition film of indium oxide in which tin, tin oxide or a mixture is doped, cannot sufficiently prevent charge injection, and it is difficult to reduce spot fogs.